The present disclosure concerns a method and a device to set the toner concentration of a toner particle-carrier particle mixture in a developer station of a printer or copier, as well as a device to develop a latent charge image on an intermediate carrier of an electrophotographic printer or copier.
The toner particles, also sometimes called “toner” for short in the following, serve for inking of the latent charge image on the intermediate carrier. The toner is then transferred from the intermediate carrier onto a recording medium, for example paper, in a further step.
For example, as carrier particles, small iron or steel granules are known. These typically have a two-fold function: on the one hand, the toner particles triboelectrically charge given blending of the mixture with the carrier particles; on the other hand, the toner particles attach to the carrier particles and, bonded to these, are conveyed to the intermediate carrier. The transport of the carrier particles to the intermediate carrier is thereby, for example, accomplished with a magnetic developer roller to which the toner particles attach. In the immediate proximity of the intermediate carrier, the electrically-charged toner particles correspondingly transfer the electrical field of the charge image to the intermediate carrier, while the toner particles remain in the developer station or are carried back to the developer station.
During the development, toner is thus removed from the developer station which must be replaced by a corresponding toner feed into the developer station. It is thereby necessary, both for the quality of the print image and for the interruption-free operation of the developer station, that the toner concentration always corresponds to a predetermined value, called a desired value in the following.
To set the toner concentration to this desired value, regulation methods are typically used in which the current toner concentration, i.e. the actual value (or a quantity dependent on this) is measured and its difference from the desired value (what is known as the regulation deviation) is minimized via suitable adjustment of a correcting variable, for example the toner feed.
To measure the toner concentration in the developer station, for example, the magnetic permeability of the mixture (which is characteristic for the toner concentration since only the carrier particles are magnetizable) can be measured with the aid of a sensor.
However, for space reasons such a sensor cannot be arranged in the section of the developer station from which the toner is actually removed for development of the charge image, as is explained in detail below using an exemplary embodiment. Instead of this, the sensor must be accommodated in what is known as the reservoir of the developer station. This is problematic since, in the print or copy operation, a toner concentration decline appears within the developer station such that the toner concentration measured in the reservoir deviates from the toner concentration in the toner extraction region relevant for the printing process. Thus the regulation is based on a falsified real value. A further problem is that the sensor measurement value is influenced by the current toner charge, which changes dependent upon, among other things, the toner flow rate. The real value forming the basis can also thereby be thus falsified.
These problems are bypassed in conventional methods in that, instead of a direct measurement of the toner concentration, a toner marking is generated on the intermediate carrier and then is scanned with a reflex light sensor or the like. A print density can thereby be determined that in turn is characteristic of the toner concentration.
This method is, for example, described in DE 101 36 259. In this, a toner marking is generated on a photoconductor, whereby this is exposed with an intensity at which the print density varies particularly significantly with the toner concentration. The toner concentration can thereby in principle be very precisely determined, especially as the concentration of the toner in a section of the developer station from which the toner has been extracted is thus actually detected.
However, the toner concentration measurement with the aid of a toner marking is indirect, inasmuch as the print density of the toner marking is still dependent on, aside from the toner concentration, a series of further quantities. Belonging to these quantities are, for example, the exposure intensity of a character generator, the degree of the electrostatic charge of the toner, the intensity of the charge of the intermediate carrier and the magnitude of the voltage between developer roller and intermediate carrier. The toner concentration can only then be reliably determined from the toner marking when all of these quantities assume a known, constant value.
However, when one or more of these quantities change without being noticed, for example as a result of a defect in the device, a false real value is supplied to the controller of the concentration regulation. This can, for example, lead to toner being continuously supplied to the developer station until this clogs, or to no toner at all being supplied over a longer period of time and the toner concentration continuously decreasing, whereby it can lead, for example, to a charge arc-over between intermediate carrier and developer roller because the electrical resistance of the mixture decreases with the decrease of the (electrically non-conductive) toner. In both cases, it can lead to severe damage to the developer station. For reasons of operational safety and monitoring capability of the system, a direct concentration measurement is thus preferable.
A further problem in conventional methods for regulation of the toner concentration is that the equalization of the toner concentration to the desired value happens relatively slowly because the regulation amplification must be kept relatively low. A too-high regulation amplification leads to an unstable regulation behavior, an increase of the interference susceptibility and poorer guidance behavior.
DE 199 00 164 A1 shows a method and a device for regulation of the toner concentration in an electrophotographic process. Two operating states are provided therein. In one operating state, a toner marking is generated on the intermediate carrier, the density of the toner marking is scanned and the toner marking is removed again from the intermediate carrier. The scanned toner marking value is used for regulation of the toner concentration in the developer station and, for example, influences a toner concentration desired value or a regulation threshold. In the other operating state, the information to be printed in the intermediate carrier is generated as a toner image and is transfer-printed onto a recording medium. In this other operating state, the toner concentration in the developer station is detected with a toner concentration sensor and it is attempted, via a corresponding return conveyance, to maintain a constant toner concentration in the developer station. As an alternative to regulation of the toner concentration with the aid of the toner concentration sensor, it is proposed to control the toner supply quantity via estimation of the toner consumption value.
DE 196 31 261 A1 shows a device for use in an electrophotographic apparatus, with a first regulation device that determines a desired value for the toner concentration in a developer station using the blackening of test markings and a second regulation device (downstream from the first regulation device) that regulates the toner concentration in the developer station based on this desired value. The second regulation device has a sensor to determine the toner concentration in the developer station and, dependent on the measured toner concentration, generates a toner refilling signal that can be optionally modified by a signal that corresponds to a toner consumption value.
In none of these documents is the problem dealt with that the toner concentration measured at the installation point of the sensor could deviate from the toner concentration at the location of the toner extraction.
Further related prior art is to be learned from the documents DE 41 37 708 C2, U.S. Pat. No. 5,353,102 and JP 03045973 A, JP 3 045 973 and U.S. Pat. No. 6,173,134.
A method to control the image density in an electrophotographic printer or copier is disclosed in U.S. Pat. No. 6,404,997 B1. In this method, the toner concentration at the developer station is calculated from a measured toner concentration and a dynamically programmed delay value. The calculated toner concentration is used to control the electrostatic developing fields.